1. Field of the Invention
This invention relates to electrophotographic recording media that can be imaged in copiers and printers, particularly high-speed color copiers and printers. The imaged media are especially suitable for use as overhead projector transparencies.
2. Description of the Related Art
In recent years, there has been an increase in the number of color copiers, color laser copiers, and color laser printers installed in offices and schools. The availability of improved, low-cost copiers and printers has fueled this expansion. Today""s machines are capable of producing high-quality multicolored images and text on paper and film media at high speeds. Offices and schools use these machines to print images on transparency films for overhead projector presentations. Such transparencies are commonly referred to as OHP transparencies in the imaging media industry.
For example, Xerox Docucolor 40 is a color copier capable of producing high-quality multicolored images on paper media with an average output speed of about 40 copies per minute. For OHP transparencies, the copying speed can be as high as 15 copies per minute.
Today""s commercially-available OHP transparencies used in electrophotographic processes consist of a polymeric film substrate (e.g., polyethylene terephthalate film) coated with one or more layers of organic materials that are receptive to a toner composition. Toners contain a binder resin and toner particles. An image is fixed onto the transparency by electrostatically transferring and fusing the toner to the toner-receptive coating. Among other factors, surface conductivity of the toner-receptive coating is important in obtaining complete toner transfer and fusion. If surface resistivity is too high, the toner may not completely transfer resulting in imaging defects such as non-uniform colors. If surface resistivity is too low, white spots and deletions in the images can occur especially in highly humid environments. In addition to enhancing image quality, the toner-receptive coating also helps improve the transparency""s copier/printer feeding performance, scratch and scuffing resistance, and mechanical strength. It should be noted that in contrast to conventional coated transparency films, the image quality and feeding performance of raw (i.e., non-coated) transparency films are poor.
As the output speed of color copiers and printers has increased, manufacturers of copiers and printers have raised the fusing temperature to ensure complete fixing of the image to the transparency sheet. Due to the high fusing temperatures in conventional copiers or printers, a transparency sheet develops a high electrical charge as it advances through the machine and is imaged. A transparency sheet in the feeding tray may become electrostatically attracted to the sheet advancing through the machine and move towards the advancing sheet causing the machine to jam. Further, when the transparency sheets are imaged and deposited on top of each other in the output tray, static electricity builds up between the stacked sheets. The imaged sheets may cling to each other making it difficult for an operator to jog and handle the sheets.
The industry is constantly attempting to develop new OHP transparencies. For example, Katsen et al., U.S. Pat. No. 5,939,193 discloses a transparent recording sheet useful in producing electrophotographic images for overhead projections. The recording sheet comprises a transparent polymeric base and imaging layer. The imaging layer comprises at least one resin and at least one transparentizer. The patent discloses that it is most preferred to use a phenoxy resin and a polycaprolactone resin in combination for the imaging layer.
Song et al., U.S. Pat. No. 5,989,686 discloses a color electrophotographic medium containing a polymeric base film having a toner-receptive coating on a side thereof. The toner-receptive coating comprises a) a low molecular weight toner-compatible resin segment such as bisphenol Axe2x80x94epichlorohydrin-based epoxy resin, b) a high molecular weight thermoplastic resin segment such as polyvinyl chloride or polyvinylidene chloride, c) polymeric particulate, and d) an anti-static agent. The 686 Patent discloses that the non-imaging side of the base film is preferably coated with a polymeric anti-static coating.
Some conventional OHP transparencies consist of a film substrate attached to a paper backing. The front (imaging) surface of such films may be coated with a variety of materials, but the back (non-imaging) surface does not possess a coating. Although some conventional OHP transparencies perform adequately in low-speed color copiers and printers, the industry is demanding transparencies that will perform well in high-speed color copiers and printers. The transparencies should be capable of forming high-quality multicolored images at high feed and output rates with minimum jamming problems. After imaging, an operator should be able to easily jog and handle the transparency sheets with minimum static-electricity problems. The present invention provides such transparencies.
The present invention relates to an electrophotographic recording medium comprising: a) a film substrate having a front surface (i.e., imaging surface) coated with a toner-receptive layer comprising about 55 to about 95% by weight of a hydrophobic toner-compatible polymer, and about 5 to about 45% by weight of a hydrophilic toner-compatible polymer based on total dry weight of the toner-receptive layer, and a back surface (non-imaging surface) coated with a polymeric layer comprising a material selected from the group consisting of particles, anti-static agents, and mixtures thereof; and b) a paper backing adhered to a portion of the back surface of the film substrate, wherein each surface of the paper backing has a Sheffield Smoothness greater than 70 Sheffield Units (SU).
Preferably, the surface contact angle of the coated front surface is no greater than about 35 and more preferably no greater than 30. Preferably, the toner-receptive layer and polymeric layer are prepared from aqueous coatings.
Suitable hydrophobic toner-compatible polymers include, for example, acrylics, bisphenol Axe2x80x94epichlorohydrin based epoxy, polyvinyl chloride, polyvinylidene chloride, and a hydroxyl modified copolymer of vinyl chloride and vinyl acetate. Suitable hydrophilic toner-compatible polymers include, for example, polyethylene glycol, polyethylene oxide, polypropylene oxide, and poly(2-ethyl-2-oxazoline).
In addition, the toner-receptive layer may comprise particles preferably in an amount of about 0.1 to about 5% by weight based on total dry weight of the toner-receptive layer. Suitable particles for the toner-receptive layer include silica, calcium carbonate, kaolin, aluminum hydroxide, starch, polystyrene, poly(methyl methacrylate), polyurethane, polyethylene, polyolefin waxes, polytetrafluoroethylene, and mixtures thereof. The toner-receptive layer may further comprise anti-static agents preferably in an amount of about 0.1 to about 10% by weight based on total dry weight of the toner-receptive layer. Suitable anti-static agents include cationically or anionically conductive polymers.
The polymeric layer coated on the back surface of the film may comprise particles such as silica, calcium carbonate, kaolin, aluminum hydroxide, starch, polystyrene, poly(methyl methacrylate), polyurethane, polyethylene, polyolefin waxes, polytetrafluoroethylene, and mixtures thereof. In other embodiments, the polymeric layer comprises anti-static agents such as cationically or anionically conductive polymers. The polymeric layer may also comprise a mixture of the above-mentioned particles and anti-static agents. Preferably, the amount of anti-static agent is in the range of about 0.1 to about 10% by weight, and the amount of particles is in the range of about 0.1 to about 5% by weight based on total dry weight of the polymeric layer.
Preferably, each surface of the conductive paper backing has a surface resistivity of less than 100xc3x971011 ohms/square, and it is more preferably in the range of about 1xc3x97101 to about 50xc3x971011 ohms/square. The paper backing may have a Sheffield Smoothness of at least 100 SU and preferably has a thickness of about 2 to about 7 mils. A vellum white paper is a particularly preferred paper backing.
The film substrate can be a transparent, translucent, or opaque film and typically has a thickness of about 1 to about 10 mils. A transparent polyester film is particularly preferred.